StructurizeCFG can't handle cases with multiple
returns creating regions with multiple exits.
Create a copy of UnifyFunctionExitNodes that only
unifies exit nodes that skips exit nodes
with uniform branch sources.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298729 91177308-0d34-0410-b5e6-96231b3b80d8
